Electrical prospecting system



y 5- 'H.M .EVJEN 2 ,375,776

ELECTRICAL PROSPECTING SY STEM Filed April 24, 1945 2 Sheets-Sheet l INVENTOR. BY f/Pza/[oiz 1% [I 2% I a I 4 May 15, 1945. H. M. EVJEN 2 3 ELECTRICAL PROSPEC'I'ING SYSTEM Filed Agril 24, 1943 2 Sheets-Sheet 2 INVENTOR.

Haaflon 1% my Patented May 15, 1945 'um'rso STATES 'PATENT OFFICE ELECTRICAL PRQSPECTING SYSTEM Haakon M. Evien, Houston, Tex, assignor to Nordel Corporation, Houston, Tom, a corporation of Delaware Application April 24, 1943, Serial No. 484,409

7'Claims. (Cl. 175-182) Other objects and advantages of the invention will be apparent as the nature of the invention is more fully disclosed.

This application is a continuation in part of my co-pending application Serial No. 477,015 flied Feb. 25, 1943, for Electrical prospecting system.

The invention will be better understood by referring to the following description taken in connection with the accompanying drawings in which certain specific embodiments thereof have been set forth for purposes of illustration.

picked-up potential measured during the current gap interval to the total earth current. The

value of this ratio depends on several factdi-s such as the nature of the switching mechanism. the disposition of the electrodes and the electrical parameters of the ground.

This system of measurement has a wide field of application and may be used for both surface and bore hole explorations. It is applicable, for example, to the system described in the Evjen Patent 2,294,395 which employs two current electrodes and two potential electrodes or to the-systems described in Evjen Patents, 2,172,557 and 2.169,685 which employ a plurality of current electrodes or to systems employing the same electrodes for current electrodes and potential electrodes as will be described.

In general, in all of these systems a current which is reversed at a controlled low frequency of, for example, from one-half to 20'cycles per second with predetermined current gap intervals at each reversal, is passed through the earth between the earth current electrodes. The pickedup potential which is obtained from the same electrodes or from separate potential electrodes is measured during the current gap intervals, that is, during the interval when'no current is being passed into theground. In operating a system of this type a plurality of readings are taken at each location under different conditions of frequency, electrode arrangement or current distribution and the results are plotted in the form of a curve from which significant indications may be obtained.

By measuringv the pick-up potential during the current gap period the masking effect of the large in phase potential due to the impressedcurrent and the effects of leakage are minimized.

A feature of the invention accordingly resides in the provision of circuits which are adapted to eflect measurements of the picked-up potential during the current ofi intervals.

In the drawings:

Fig.1 is a diagrammatic representation of a geophysical prospecting system illustrating a simplified'em'bodiment of the present invention;

Fig. 2 is a diagrammatic representation of a similar system illustrating an embodiment utilizing commutated current wherein the same electrodes are used for earth current electrodes and potential pick-up electrodes; I

Fig. 3 is 'a similar diagrammatic representation illustrating an embodiment of the invention wherein separate current electrodes and potential pick-up electrodes are employed; and

Fig. 4 is a diagrammatic representation illustrating the application of the invention to a system of measurements employing a plurality of earth current electrodes.

In the following description certain specific terms have been used for convenience in referring to various details of the invention. .It is to be understood, however, that these terms are to be given as broad an interpretation as the state of the art will permit.

Referring to Fig; 1, a current source It is connected by lines II and I2 to fixed contacts I 3 and I4 respectively of a motor driven,-doublepole, double-throw switch I! which may be of any suitable type, for example, of the type shown in Evien Patent 2,314,874, dated March 30, 1943. which shows a switch having means to adjust the timing of the contact periods and oi the change-over or gap periods. A current measuring device such as an ammeter I8 is shown as connected in the line l2.

A'potential measuring device such as a voltmeter 2ll is connected by lines 2! and 22 to fixed contacts 22 and 2 respectively of the double-pole double-throw switch 15. The movable contacts of the switchii are connected by lines 25 and 26 to earth electrodes 21 and 28 respectively. The switch I5 is adapted to be periodically reversed by any suitable driving means indicated diagram matically at 30. The electrodes 21 and 28 may be disposed along the surface of the earth, in bore holes or in any other desired manner in accord-' ance with the measurements to be taken.

In the operation of this system the switch I5 is adjusted to provide predetermined earthcurrent 'periods, gap periods, and potential pick-up -the contacts 23 and 24 are closed, the earth potential picked up by the electrodes 2'! and 28 is measured by suitable means indicated as a voltmeter 28. These measurements may be repeated with different electrode arrangements or at difference frequencies. The ratios of the readings of the ammeter I6 to the readings of the voltmeter 28 provide a significant physical quantity which is a measure of the earths characteristics.

In the embodiment of Fig. 2, provision is made for interrupting and reversing the earth current at predetermined intervals. In this embodiment the source In and voltmeter 28 are connected to the motor driven, double-pole, double-throw switch IS in the manner set forth in connection with Fig. 1 and the corresponding parts have been given the same reference .characters. In Fig. 2 the movable contacts'of the switch l5 are connected by lines 3i and 32 to stationary contacts 33 and 34 respectively of a reversing switch 35, which may also be of the type set forth in the Evjen Patent 2,314,874 referred to above.

One movable contact 36 of the reversing switch 35 is connected by a. line 25 to the earth electrode 21. The other movable contact 37 is connected by a line 38 to the variable contact 39 of a potentiometer 48. A battery 4| is connected across the potentiometer 48 and a switch 42 is provided to interrupt the battery circuit when measurements are not being taken. The mid-point of the battery 4| is connected by a line 43 to theearth electrode 28. I

The switches l5 and 32 are interconnected by driving means, indicated diagrammatically at 44, for operation in unison, with the switch 35- operating at half the speed of the switch l5 and timed toreverse its connections during alternate I reversing or gap periods of the switch I 5.

In the operation of this system, with the switch 1 35 closed in one direction the switch I5 is first shifted toxclose contacts 13 and I4 and thereby pass a current through the earth in a predetermined direction between the electrodes 21 and 28. After a predetermined current interval the switch i 5 is shifted to close the contacts 23 and 24 whereupon the potential picked-up by the electrodes 21 and 28 is applied to the voltmeter 28 for measurement. After ,a predetermined pick-up interval the switches 15. and 35 are both shifted in unison, whereupon the current fromthe source 58 passes through the earth between the electrodes 21 and 28 in the opposite direction. At the next reversal of switch l5 the picked-up potential. which is likewise of opposite direction,

. is indicated by the voltmeter 20. The cycle is thenrepeated. v V I The effect of the natural ground potential,

which constitutes a direct self-potential between the electrodes 2? and 28, is eliminated by suitable adjustment of the potentiometer 48 to introduce I a potential which balances or counteracts the self-potential. With. the contact 39 at its central position no potential is introduced bythe Q I potentiometer. -As the contact 38is'shifted in one direction or the other, however, a positive or negative bucking potential may be introduced. This adjustment maybe made when no measurements are being taken, as for example when the earth current circuit has been interrupted for a period of time. so that the only effect on the voltmeter 28 is that produced by the normal earth potential. It may also be made during operation inasmuch as the reversal of the normal earth potential by the reversing switch causes fluctuations of the voltmeter 28. The contact 88 may accordingly be adjusted to eliminate such fluctuations and any drift in the natural earth potential will be immediately perceived and can be compensated for by readjustment oi the contact 89. The position of the contact 88 on the potentiometer 48 provides a measure of the normal ground potential which constitutes a second element of the log. The impedance-measured in this way has characteristics that diner from the usual resistivity measurements. The observed value of this quantity depends on the frequency of commutation and may be plotted thereagainst to give a significant function.

In Fig. 3 the invention is shown as applied to.

a system involving separate earth current electrodes and potential pick-up electrodes. In this embodiment a source 58 of direct current is connected by means of lines 5! and 52 to the movable contacts 53 and 54 respectively of a motor driven reversing switch 55. which may be similar to the switchl5 above described. An ammeter 58 is shown as connected-in the line 52 for purposes of regulation and control. The switch 85 is driven by a suitable driving means indicated diagrammatically at 51 which i provided with mechanism for rocking the movable contacts 53 and 54 at a predetermined frequency and with predetermined contact and gap intervals.

The reversing switch 55 is provided with a pair of stationary contacts 58 which are connected by a line 59 to an earth current electrode 88 and with ,a pair of stationary contacts 8| which are connected by a line 62 to an earth current'electrode 63.

The potential pick-up circuit comprises a pair of pick-up electrodes 18 and II which are located within the field of influence of the earth current, either on the surface of the earth or vertically displaced along a bore hole for well-logging purposes. The pick-up electrode 18 is connected by a line 12 to stationary contacts 13 of a reversing switch I4 which is similar to the reversing switch 55 above described and is connected to be driven by the driving means 51 in unison with the switch 55 but with a phase displacement such that the contacts of the switch 55 are closed for definite periods while the switch I4 is open and vice versa.

The potential pick-up electrode II is connected by a line 15 to the mid-point of a battery 18. The battery 16 is connected across a potentiometer 11, a switch 18 being provided to interrupt the battery circuit when measurements are not bein taken. The potentiometer 11 is provided with a movable contact 18a which is connected by a line 19 to thestationary contacts 88 of the switch 14.

A movable contact ill of the switch I4 is connected by a line 82 to a movable contact 83 of a potentiometer 84. The movable contact 85 of the switch 14 is connected by a. line 86 to a D. C. galvanometer 81 the other side of which is connected by a line 88 to 'a second movable contact 88 of the oineter 84 to supply potential thereto. A switch I 02 is provided for interrupting the battery circuit when measurements are not being taken.

- In the operation of this system the electrodes 4 60, 63, and II are arranged in accordance with the area to be investigated, for example, along the earths surface or along a bore hole. In the latter event, one of the electrodes. for example, electrode 60, may be located at the earth's surface and the electrodes 03, 10 and TI may be spaced vertically down the bore hole as is customary in well log ing.

A current is passed through the earth between th electrodes 60 and 63 which has a definite value and frequency, the value being dependent upon the source and the resistance of the earth current circuit and the frequency being dependent upon the frequency of operation of the reversing switch 55. This current is in the form of a flat topped wave having a predetermined current gap at each reversal. Referring to a complete cycle at 360". the reversing switch may be adjusted for a- 120 current gap and a current supply interval during each half cycle.

The earth potential picked up by the electrodes 10 and H, which has a frequency corresponding to the frequency of the earth current, is rectified by the reversing switch 14, which is operated in unison with the switch 55 but is displaced in timing-0r phase relationship so that the contacts of the switch 14 are closed duringthe current gap intervals of the switch 55 and vice versa. For example. the switch 14 may be adjusted to have 120 gap intervals and 60 pick-up intervals during each half cycle, the 60 pick-up intervals being timed to occur during the 120 current gap of the switch 55, and the 60 current supplyinterval of the switch 55 being timed to occur during the 120 gap period of the switch 14. In particular the switches are adjusted to operate 90 out-of-phase. I

The out-of-phase or imaginary component of the picked-up potential is thus rectified by the switch 14 and is applied as a uni-directional potential to the D. C. galvanometer 81; A balancing potential is also applied to the galvanometer 81 by means of the potentiometer 04, the contacts 03 and of which are adjusted so that the potential drop therebetween opposes and balances the potential applied to the galvanometer 01 from the pick-up electrodes 10 and 1|. A null balance is thus obtained and the value of the picked-up potential can be determined by the setting of the contacts 83 and89 of the potentiometer 04. At this point of zero reading the time average of the current flowing in the potential circuit is zero. Consequently, the major effects of line resistance, contact resistance and the like are eliminated.

Any residual earth current which would tend to produce a fluctuation of the galvanometer 01 is balanced out by suitable adjustment of the potentiometer 11, which corresponds to the potentiometer 40 of Fig. 2, and may be adjusted in a similar manner. The adjustment is such as to eliminate fluctuation of the needle of the galvanometer 81.

After the above adjustments have been made a series oi readings may be taken at different frequencies by a suitable operation of the control mechanism 51, and the variations with respect to frequency are indicative of the characteristics of the earths strata.

During the periods of measurement the earth current as indicated by the ammeter 55 should remain as constant as possible, or the reading of the ammeter 56 may be taken when a balance has been obtained on the galvanometer 01 and the ratio of the earth current to the imaginary component of the picked-up potential may be used for the determinations.

In the embodiment of Fig. 4. the invention is shownas applied to the multi-electrode method of exploration. In this embodiment a source of direct current is connected by lines ill and 02 to fl-xed contacts 93 and 94 respectively of a reversing switch 05 which is similar to the reversing switch 55 of Fig. 3. An ammeter 06 is shown as connected in the line 02 for purposes of measurement. One movable contact of the switch 05 is connected by a line 08 to an earth current electrode 99. The other movable contact of the switch is connected by a line I00 to the midpoint of a network comprising a pair of variable resistances IM and I02, 9, fixed resistance I03 and a potentiometer I04 connected in series be tween earth current electrodes I05 and I05. A galvanometer I01 is connected across the'fixed resistance I03 to a variable contact I00 on th potentiometer I04.

A galvanometer H0 is connected by lines III and H2 to fixed contacts H3 and H4 respectively of a reversing switch II5, which is similar to the reversing switch 14 of Fig. 3. One movable Contact of the reversing switch H5 is connected by a line IIG to an earth potential electrode Ill. The other movable contact of the switch H5 is connected by a line IIB to a contact M0 on a potentiometer I20 across which a battery I2I is connected, The midpoint of the battery |2I is connected by a line I22 to the other earth potential electrode I23. A switch I24 is provided for interrupting the battery circuit when measurements are not being taken. The switches 05 and H5 are driven in unison, but at a 90 phase displacement by suitable driving means I25 and are adjusted for alternate operation as set forth in connection with switches 55 and 14 of Fig. 3.

In the operation of this embodiment the variable impedances IN and I02 are so adjusted that zero potential exists between electrodes 1 and I23 as indicated by the galvanometer IIO.

When this state exists the Sliding contact I00 of the potentiometer I04 is adjusted to give a zero reading on the galvanometer I01. This gives a measure of the ratio of the current in the two branches represented by the electrodes I05 and I00. If, for example, the scale of the potentiometer I04 is divided into a hundred divisions, the setting for a null reading provides a direct reading on a percentage basis of the ratio 01 the current through the electrodes I05 and I05. .A set of readings taken under diflerent conditions of frequency, electrode spacing etc., may be plotted for determining the characteristics of the earth.

Although certain specific embodiments of the invention have been shown for purposes of illustration, it is to be understood that the invention is not to be restricted thereto, but that various changes and modifications may be made therein as will be readily apparent to a person skilled in the art. The invention is to be limited only in accordance with the scope of the following claims.

What is claimed is:

1. The method of electrical prospecting which comprises passing a current from a direct current source through the earth between one current electrode and a plurality of other spaced current electrodes, periodically interrupting and reversing said current at a contolied low frequency with controlled current gap periods at each interruption, picking up during said current gap periods the ground potential between a pair of spaced pick-up electrodes, adjusting the current distribution between said current electrodes to nullifysaid ground potential and measuring the ratio of the currents supplied to the various current electrodes to obtain said null balance,

2. A system for electrical prospecting comprising an earth current circuit including a source of direct current, a pair 01' spaced earth electrodes and a. reversing switch connected between said source and said electrodes, means actuating said reversing switch periodically to interrupt and-reverse the earth current at a predetermined controlled low frequency with a predetermined current gap period at each interruption, a potential circuit including a D, C. potential measuring instrument, means picking up the potential difference between spaced points within the field of influence of said earth current, a reversing switch in said potential circuit timed to convert said picked-up potential to a unidirectional potential comprises passingthrough the earth an input current in the form of a series of similar pulses, successive pulses being in opposite directions and being spaced by a gap period during which no current flows, said gap periods and said pulse periods together forming a repeated pattern of controlled frequency, and picking up the potential between spaced points within the field of influence of said input current during the current gap periods, separating the component of said picked-up potential due to the input current from other components, and measuring said first component.

5. The method of electrical prospecting which comprises passing through the earth an input current in the form of a series of similar pulses,

successive pulses being in opposite directions andpotential between spaced points within the field of influence of said earth current during the current gap periods, reversing said picked-up potential to produce a unidirectional potential, and supplying the unidirectional potential to a direct current measuring device, and eliminating the effect on said measuring device of picked-up potential components other than the component due to said input current.

6. A system for electrical prospecting comprising an earth current circuit including spaced earth current electrodes, timed means supplying to said electrodes for passage through the earth a current in the form of a series of similar pulses, successive pulses being in opposite directions and being spaced by a gap period during which no current flows, said gap periods and said pulse suited to actuate said measuring instrument, conperiods together forming a repeated pattern of controlled frequency, probe electrodes in the earth within the field of influence of said earth current, and a measuring circuit connected to said probe electrodes including means to separate the alternating component of the picked up potential from the unidirectional component thereof, a circuit responsive only to said first component and a measuring device connected in said last circuit to measure said first component.

v7. A system for electrical prospecting comprising an earth current circuit including spaced earth current electrodes, means supplying to said electrodes for passage through the earth a current in the form of a series of similar pulses, successive pulses being in opposite directions and being spaced by a gap period during-which no current flows, said gap periods and said pulse periods together forming a repeated pattern of 'controlled frequency, probe electrodes in the earth within the field of influence of said earth current, a potential measuring circuit connected to said probe electrodes including a D. C. potential measuring instrument, and a reversing switch timed to close said measuring circuitonly during current gap periods and to convert the picked-up potential into a unidirectional potential suited to actuate said measuring instrument.

HAAKON M. EVJEN. 

